Ray, you seem to have completely misunderstood my question, which in particular was NOT about deciding which format to use for a certain situation. Let me try again. Suppose that you have set up to take a photo with a certain format, and have chosen your camera position, focal length, aperture setting, ISO, shutter speed etc. to give what you consider to be the most desirable artistic result, with no consideration given to mundane issues like the size, weight, cost or availability of the lens needed. Then you are asked to do it instead with a camera of a smaller format, but with an equally unlimited lens options. How would you change your "ideal" choice of lens and settings?

My expectation is that, with the same artistic objective, you would want to keep the same perspective, depth of field, and either want the same shutter speed or be open to any higer speed too, and so would keep the same camera position, adjust focal length in proportion to image sze, keep the same aperture diameter, and so use a proportionately larger f-stop. Speed choices are less clear; perhaps the same ISO and thus the indicated higher shutter speed, perhaps keep it the same or closer to what you chose for the larger format with a lower ISO setting or possibly an ND filter.

If not, please explain why you ideal, preferred composition would instead change in perspective, depth of field or such.

P. S. you allegation about my lens envy is quite bizarre, since you are the only one bringing your available lens limitations into the discussion of compositional choices. Quite sincerely, my biggest single reason for having balked at buying a Canon 10D (which would work with my existing Canon 28-105 lens and a few accessories) is the poor lens choices for the type of photography that I do. Conversely, my main reason for currently favouring the Olympus E-1 is that its 14-54 f/2.8-3.5 is by far the most attractive current or announced choice for my particular purposes, offering FOV comparable to 25-100mm in 35mm format for the traditional print shapes I prefer; 4x5 to 3x4. Comparing to the 10D, the comparision is about 16-63 for FOV, between f/2.8-3.5 and f/4-5 for speed (the slower range allowing for the extra stop or so of usable pushed sensor speed on the 10D), f/4-5 for minimum depth of field.

Let me give you a concrete example from recent experience. Many art galleries, museums, historic homes etc have rules about photography. Sometimes no photography at all is permitted. Sometimes photography is permitted but no flash or tripod allowed.

In such circumstances one struggles with the competing constraints of shutter speed and DoF. One usually ends up with a compromise, or no shots at all; perhaps f5.6 at 1/30th is the best one can do.

Enter a nice gentleman like yourself who hands me a Leica Digicam; do I now use F2 at 1/250th? Almost certainly not. I might well stick with the same f stop and shutter speed, thus lifting those constraints, achieving greater DoF and a more appropriate shutter speed for a hand held shot with the shorter lens.

This is a good one. You choose a low light situation, then extoll the virtues of the more light-sensitive camera. Of course, we get it but how does that matter in the discussion?

The point of BJL's scenario is to free you from any concerns that may limit your artistic choices. It is then that you can freely choose to duplicate your photographic choices in another format. Once you do that, you will see that DOF is not dependent on sensor size.

I can't believe this: five (5) pages of supertech mumbo-jumbo hyperbole about DOF without a single reference to the formula or to the almighty critical role of the "circle of confusion"!

Yes, it's weird isn't it! I get the impression some people are just arguing for the sake of arguing, and it's probably now time for me to bow out of the discussion.

My interest in this thread began with a comment from BJL who suggested the smaller format digicam has greater potential than we sometimes imagine. Here is a quote that caught my attention.

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The key here is noting that smaller formats can typically be used at lower f-stop's, and hence at lower ISO settings, off-setting their weakness when compared at equal ISO, and that bigger sensors can only provide their potentially greater image quality when provided with enough light to avoid underexposure.Larger sensors can give higher image quality when provided with enough light, but an "information theoretic" approach suggests that with a given, limited amount of light, any sensor big enough to take in all the available light without highlight blow-outs, combined with a fast enough lens, can give "image detail" as good as a larger sensor, which has to be unerexposed with that same light levels. Below a certain moderate level of total available light, even 2/3" format can roughly match any larger format. However the lens speed of the Digilux 2 is not enough to fully take advantage of this: I dream of a future f/1 zoom lens in this very small format.

I was really mainly interested in exploring this idea but in the process seem to have upset, enraged and confused some people by indirectly associating format size with DoF.

I don't generally find it useful in my photography to calculate the diameters of 'circles of confusion' or the physical size of apertures at a particular f stop, but thanks for your formulae. I'm sure some people will find them useful.

For example, the DOF of the "cropped" image from a Canon 10D with a 50mm lens focused at 3m at f/8 is exactly the same as the DOF of the "full" image from a Canon 1Ds with the same lens, focus setting, aperture and camera position. Of course, the image content from the 1Ds is not the same because the "crop" is gone, but the DOF of that wider image will not change because none of the original DOF variables will have changed!

That is so if you keep the circle of confusion the same. This is turn might make sense if you make a print in which the same subject in the image comes out the same size on the print, which is then viewed from the same distance. That way, the circles of confusion as they appear on the print are the same size, and with the same viewing distance, have the same effect on perceived sharpness.

But if you make the same size print for the "cropped, mildly telephoto" 10D image as for the "normal FoV" 1Ds image, the greater magnification from sensor to print will make the printed circles of confusion bigger, and so give less DoF in the image from the 10D than in the image from the 1Ds. The hitch is that the "angular CoC" in your formulas assumes a lens of "normal" angular FoV, which the 50mm ceases to be when used on the 10D.

To maintain the same DoF in this second comparison, the DoF scale for the smaller format needs to be computed on the basis of a smaller CoC.

(There is more about this in my very long new post in a new thread, though I omitted all formulas, hoping that everyone here understands their main conseqences by now! That might not be fair to newcomers who missed the previous long DoF discussion.)

One should also bear in mind that, by coincidence, 35mm is also very close to the length of the image so the term does not seem totally absurd even if one were not aware that it actually referred to the total width of the film.

2/3rds on the other hand seems to have no connection whatsoever with any dimension of the 2/3rd's format, not the height, not the width, not the diagonal. So what we have here is tradition supplanting common sense.

I suspect the real reason the term is still used is to obscure the fact that a 2/3rds sensor is really very tiny. It's not good for marketing purposes to let it be widely known that the imaging chip in the camera you're flogging has only 1/16th the area of the standard miniature (35mm) camera that people are familiar with.

It would be no exaggeration to describe these point and shoot digicams as sub-miniature cameras.

True! On the other hand, if you go back to the beginning of this thread where there's a link to the relevant page on the Leica site, you'll find the following statement:-

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The 2/3-inch CCD sensor is very large for its resolution of 5 million pixels.

.... and there's no mention of the actual dimensions of the 2/3rds format. In fact it took me quite a bit of searching on the net to find those dimensions. Of course, I ended up at dpreview where I should have gone in the first instance.

I'm not talking about conspiracy theories here but rather a disincentive to get rid of a meaningless term because of its marketing advantage. I'm sure there are lots of potential buyers out there who equate pixel count with quality and resolution and who are attracted to the idea that such a small 5MP camera has almost as many pixels as a Canon 300D.

The only meaning I could get out of the brief dpreview explanation is that 2/3rds expresses the relationship between the sensor size and the size of the image circle thrown by the lens. Now that's really useful information to have.

A bit more briefly, about the EVF, since this is a developing concept that the Leica Digilux 2 will put to the test. For this, one has to accept the idea of using a sensor in 2/3" format or smaller; this should not be so hard since the overwhelming majority of digital cameras have such small sensors, including some that have produced quite good and even professional images. Given that the sensor is so small, the EVF is probably already the best option, or will soon be: consider the alternatives.a) peep-hole viewfinders, including true range-finders. These have poor compatability with wide ranges of focal lengths, from either ultra-wide ranging zooms, inerchangable lenses or use of supplementary lenses on digital cameras. None currently works with sup lenses since the sup lens blocks the viewfinder, and as Lecia has pointed out, once you put a sufficiently big fast lens on the camera, the peep-hole VF again gets obstructed. They also lack depth of field preview, and various other "TTL" virtues. LCD on the back. The problems are well known: they require holding the camera in an unstable "arms' length" position, and give a small image that is hard to see in bright light.c) Optical TTL VF. The image is currently constrained to be rather small or dim or both with such small formats. If the image were made as large as in 35mm format VF with the f/2 lens of the digilux, brightness would be as for an f/8 lens in 35m format. The mythical f/1 zoom that I fantasize about in my previous post would improve this to a tolerable but not great f/4 equivalent brightness. That would still leave the other disadvantages of SLR's, like bulk and noise.

Compared to all this, a high resolution EVF's with zooming at the focus point to aid manual focus are probably the future for high end compact digital cameras. Actually I believe that every current 2/3" format digicam has an EVF except the Olympus E-20 which is approaching end of life, so I am not sticking my neck out much with that prediction.

Ray,>I see you haven't lost your enthusiasm for the smaller format.Not for "limited light photography" as I envision for cameras like the Digilux 2; and the dominance of 35mm over all larger film formats for low light and high speed photography shows that I am far from alone.

>The smaller the format, the smaller the maximum print size in>relation to a given standard of acceptable sharpness and the>narrower the range of DoF settings (for acceptable sharpness).>I see no way around this.

Agreed, with one crucial qualification: shifting to a larger format only fully gives this advantage when it allows one to gather more light, through some combination of larger aperture diameters (less DoF etc.) and longer exposure times that would not possible in the smaller format because it would lead to overexposure. In "signal to noise ratio" terms, a larger format benefits when it gathers more signal, but if it is limited to gathering the same signal as a smaller format, gathering that light over a larger sensor area collects more sensor noise along with the same signal, and so it is worse off. The larger sensor is not any worse off though, since one can instead use the same focal length as with the smaller format, forming the same size of image, and then crop, discarding the extra noise at the unused photosites. Only cost and weight constraints, and perhaps sensor read-out speed, wil fundamentally prevent a larger format from performing at least as well as a smaller one.

I guess you’ll all be shouting at me in a moment, but this stuff is photography 101! Reading through this ant the other thread on this subject gives me the impression that there’s some basic photographic foundation knowledge is missing, and in turn a lack of common vocabulary.

In the UK (a place known for good photographers) they teach this stuff with this http://www.amazon.com/exec....s=books book - Basic photography by M. Langford. I would recommend it, so you can understand what effects the lights path from subject to the focal plane. It is also impossible to explain this without pictures of the light path.

And before you all get mad at me, remember it is only about the final picture, and it is not until the pictures are produced can we have an opinion on will this Leica be good enough or not!

Those people don't necessarily fail to realize that the physical aperture has changed, but perhaps they like shooting at 1/60....In the real world, a small sensor camera will have difficultly restricting DoF. Maybe they will release a F/0.7 lens on a small-format camera in the future with ISO 16 so you can both limit DoF and have a slower shutter speed, but I have my doubts.

Now we are getting somewhere.

I) Limitations of available lenses.To take your second point first, I agree, with some qualifications:a) Current digital cameras with sensors distinctly smaller than 35mm format have difficulty restricting DoF, due to their small actual maximum apertures, and for sufficiently small formats (2/3" and below) this might always be the case, if only for the market driven reasons that for small, convenient cameras, seeking more DoF is far more common than seeking less, so the makers' choice of maximum aperture is driven mostly by speed rather than DoF considerations: f/0.7 will never be in much demand. Current "smaller than 35mm format" interchangable lens DSLR's are more limited in shallow DoF options at normal to wide focal lengths by current lens options. Again, it is likely that there is not enough demand for less DoF in wide angle, so aperture options wil be driven by speed needs, which are not very great in wide angle: the trend after all is f/4 only rather than f/2.8 even for high quality lenses, taking advantage of the greater usable ISO speed of DSLR's compared to film to reduce cost and weight.

On the other hand, into the telephoto range, the maximum aperture available does not increase much with focal length (or, the minimum f-stop declines: f/1.2, f/1.4, f/2, f/2.8, f/4 ...) so for equivalent FOV, DoF options are not much dfferent between APS and 35mm DSLR's, and both can generally get shallower than in MF or LF, where there is less in the way of telephoto lenses with huge maximum aperture diameters. Further, in this realm, more speed is always in demand, so I can easily imagine in the future something like a 300/2 as an APS counterpart of 400/2.8 in 35mm format.

II) Shooting at a given shutter speed.Now to the first point, of wanting to shoot at a given shutter speed. If this is for the sake of freezing subject of camera motion, it is only a lower limit on shutter speed, so the only effect is to force larger formats to give up some DoF; those issues to do not prevent the user of a smaller format increasing speed to compensate for the greater speed at a given DoF. If the issue is having a long enough exposure time to produce some effect like deliberate motion blur, all it takes with the smaller format is some combination of changing to a lower ISO setting (which gives better image quality anyway) and if that is not enough, adding an appropriate ND filter.

Either way, nothing about shutter speed choice prevents the smaller format from achieving DoF as shallow as with a larger format; given appropriate lenses, smaller formats just have a few more options.

I did not. I said that fixing f-stops while varying focal lengths across formats was invalid if you were trying to compare DOF of the two formats........

This reminds me a bit of the sorts of arguments I used to have with my ex wife. "But you said .... "No I didn't, I said...." and so on. I wish I'd had a tape recorder running continuously.

Fortunately on this forum we have a record of everything written. This is exactly what you wrote:-

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You need to understand that holding the f-number constant across formats is meaningless and bogus. Once you figure that out, you'll realize the DOF claim is wrong.

This statement is clearly, definitely and demonstratively wrong.

Holding the f-number constant across formats is neither meaningless nor bogus. The physics that determines that DoF will remain the same for a lens of a given focal length and given f stop irrespective of camera format, is the same physics that determines that an increase in DoF will result if the focal length is shortened and the f stop kept constant. Every photographer should know this.

It is also sometimes of practical significance to LF photographers who use a shift mechanism for architecture shots. As I'm sure you already know, to use tilt and shift requires that the image circle is significantly larger than the film format. In order to achieve this larger image circle, LF photographers will sometimes use a lens designed for a larger format (say 8x10) with a smaller format body (say 4x5). They have to be aware that in doing this, DoF does not change for the same f stop. This is not meaningless or bogus.

Now you might argue that this is obvious, and I would agree that it is obvious to those who have thought about it and already know it. But it's not necessarily obvious to those who have not thought about it.

In any case, I wasn't aware this discussion was about the relative transparency of various statements. None of my statements in this thread are incorrect (as far as I know. If any are, please enlighten me )

Please Ray. You have no intention of holding focal length constant (or anything else for that matter). The thrust of your endless arguments is that you reserve the right to vary every parameter as it suites you, then claim that small sensors have a DOF advantage.

Now you're beginning to make sense. Yes I do reserve the right to vary every parameter as it suits me, and it is perfectly true that small sensor cameras have a DoF advantage as well as an exposure advantage in low light situations.

I don't generally find it useful in my photography to calculate the diameters of 'circles of confusion' or the physical size of apertures at a particular f stop, but thanks for your formulae. I'm sure some people will find them useful.

Nor do I, Ray. These things have already been done for us by the manufacturer's data included with new lenses, and we can often preview actual DOF within our cameras.

The point is that the only variables in the DOF formulas are the focusing distance, the diameter of the aperture and the size of the circle of confusion (which is a funtion of the focal length of the lens). Thus, these are the only factors that can possibly have any direct affect on DOF in any given configuration. Change the configuration, e.g., to a larger film format that requires a different lens, and one or more of these variables will have to change accordingly in order to change the DOF.

For example, the DOF of the "cropped" image from a Canon 10D with a 50mm lens focused at 3m at f/8 is exactly the same as the DOF of the "full" image from a Canon 1Ds with the same lens, focus setting, aperture and camera position. Of course, the image content from the 1Ds is not the same because the "crop" is gone, but the DOF of that wider image will not change because none of the original DOF variables will have changed! Simple, huh! ::

BJL, Having done many years of professional action photography I have to agree that 35mm has a lot of advantages and delivers the quality. The only reasons not to use a format are quality and/or ergonomics.

IMHO the smaller formats extra depth may be more of a problem than a benefit. Seperating subject from some distraction is an often challenging requirement.

And f5.6 was just a choice, I'm often stuck on f2.8 for exactly the reason you describe - no light! Yet for most this discussion is just a big so what!

The use of inches is anachronistic but not so surprising, being a product of American television engineers in the 1950's or so; it is more interesting to me that some decades earlier, Thomas Edison and George Eastman went metric for the 35mm film format. But no-one has answered my question about the significance of the length 35mm.